Cultured normal rat retinal pigment epithelium retains the capacity to phagocytize photoreceptor outer segments, whereas RCS rat pigment epithelium is deficient in this capacity. This experimental system is being used to study the mechanism of outer segment phagocytosis by the pigment epithelium. The interaction between the pigment epithelium and outer segments is being studied in both normal and RCS rat pigment epithelium by selectively modifying surface components of outer segments and pigment epithelium and by varying the extracellular environment. The effects of inhibitors and stimulants of phagocytosis are being investigated. Experiments are being done to compare the binding of outer segments by normal and RCS pigment epithelium and to stimulate phagocytosis by RCS pigment epithelium. Should the specific defect in RCS rat pigment epithelium be found, the study will provide an important precedent for understanding an underlying cause of inherited retinal degeneration.